Sims 3 won't play on Mac

Hello. I hope this is the write place to put this question as I've never been on this forum before,

I got the Sims 3 last year and shared an account with my sister. I got a new computer and tried playing the Sims 3 on it, using the same account. (I had read that you can use the same account on multiple devices but only one device may be on the game at a time.) I play the game through Origin.

I wasn't able to get the game going using the same Origin account, so I deleted the files from my computer. I made my own Origin account and bought the game again. The launcher loads fine, but when I click the 'Play' arrow I get a message that says "Cannot start the process while the game is already running. Please quite the game and try again." But the game isn't already running or anything.

I've uninstalled and redownloaded and installed the (newly purchased) game twice. I'm on my new account on both Origin and the Sims 3. I've done everything I can think of to solve the problem and yet, I continue to get the same error message.

Hopefully somebody has had (and solved) this problem. If I didn't write enough information just let me know and I could add some. Thanks!

@Bluebellflora   please and thank you in advance